The present invention relates to an improvement in an aspherical lens for an indirect ophthalmoscope
One type of indirect ophthalmoscope employed in medical treatment of eyes comprises, as shown in the FIG. 1, a condenser lens 2 and an aspherical lens 3 for causing a bundle of rays omitted from a light source 1 to converge in the vicinity of the pupil of an eye 4 being examined. As illustrated in FIG. 2, an image of the fundus of the eye 4 is focused by the optical system of the eye being examined and the aspherical lens 3 on a position 5 in space, the focused image being observed by the examiner through an eyepiece 6. The aspherical lens 3 may be utilized as a magnifier to examine an object 8 at an anterior portion, the object 8 being observed as a virtual image 9, as shown in FIG. 3.
Conventional aspherical lenses have spherical aberration and coma relatively well compensated for or corrected so as to be usable in an illumination optical system, and also have relatively well corrected for distortion for use as a magnifier optical system. Spherical aberration and coma under the conjugate condition for pupils (in which the pupil of the eye 4 being examined and that of an eye 7 of the examiner are in conjugate positions) when used as an observation optical system, can also be compensated for when the spherical aberration and coma for the illumination system have been corrected. However, the astigmatism of the lens used as the observation optical system has not been corrected to a substantial degree. When the fundus image is observed by the examiner with the unaided eye, as shown in FIG. 2, the ray of light is limited by the pupil of the examiner's eye 7. Because of the accommodation of the examiner's eye 7 and the increased depth of field, thre has been no need for substantially correcting the astigmatism. Where an optical system utilizing an indirect ophthalmoscope is used, as illustrated in FIG. 4, to form a fundus image at a position 5 in space, and such a fundus image is photographed by a film 12 through a semitransparent mirror 10 and a photographic optical system 11, good picture quality has not been obtained with the prior aspherical lenses.